The overall goal of this project is to assess the influence of microorganisms on the fate of polynuclear aromatic hydrocarbons (PAH) in soil environments over time, in order to better determine the potential health-risk exposure to human or indigenous organisms. The microbial metabolism of PAHS can result in; (1) the accumulation of more polar by- products (that may be more or less toxic or mobile than the parent compound) and (2) the formation of by-products that can be subsequently metabolized then mineralized to Co2 (usually by other microorganisms). An alternative fate for either parent compounds or metabolites is the interaction of PAHs with soil organic matter (SOM) that may lead to incorporation into SOM via sorption processes (Walton et al., 1994). The latter has recently been demonstrated in our laboratory and others. We now propose to determine both the magnitude and mechanistic pathways of these interactions with environmental samples. If incorporation of PAHs into SOM is a significant fate of PAHs in soil, it will profoundly impact the toxic properties of both PAHs and PAH by-products. A priori, incorporation is likely to reduce toxicity. If so, this route could provide a simple, cost effective strategy for bioremediation. Experiments will be conducted to investigate which characteristics of the soil and microbial community can impact biodegradation versus incorporation. To achieve these goals, the following aims are proposed; 1. Assess the relateive rates of microbially mediated degradation versus incorporation into the soil organic carbon matrix for a range of PAHs (3, 4, 5, and 6 ring: phenanthrene, pyrene, chrysene, and benzo(a)pyrene) and their microbial degradation products. 2. Investigate the extent of incorporation of the PAHs and their degradation products into SOM and gain insight into incorporation mechanisms by using 13C-NMR to evaluate binding of 13C-PAH to SOM. 3. Estimate the impact of degradation and incorporation processes on toxicity (measured with MICROTOX) and mutagenicity (measured with MUTATOX tm). 4. Relate rates and magnitude of degradation and/or soil interactions to the characteristics of the chemical, the soil, and the microbial community. 5. Develop bioremediation strategies.